Typical Audit Trail ?

I tried to implement triggers for filling audit-trail table on this way.
Everything works fine as long as I don't update the primary key field value.
When I try to update PK value, an error occures.
The code is the following:

CREATE TRIGGER NameOfTheTrigger
ON dbo.TableName FOR DELETE, INSERT, UPDATE
AS BEGIN
declare
@type varchar(10) ,
@UpdateDate datetime ,
@UserName varchar(128)

if exists (select * from inserted) and exists (select * from deleted)
select @type = 'UPDATE'
else if exists (select * from inserted)
select @type = 'INSERT'
else
select @type = 'DELETE'

select @UpdateDate = getdate() ,
@UserName = system_user


/* this code is repeting for every field in the table*/
if update (TableName) or @type = 'DELETE'
insert dbo.AUDIT_TRAIL (TableName, FieldName, OldValue, NewValue,
UpdateDate, UserName, type)
select 'TableName', convert(varchar(20), 'FieldName'),
convert(varchar(1000),d.FieldName), convert(varchar(1000),i.FieldName),
@UpdateDate, @UserName, @type
from inserted i
full outer join deleted d
on i.PrimaryKeyFieldName = d.PrimaryKeyFieldName
where (i.FieldName<> d.FieldName or (i.FieldName is null and d.FieldName is
not null) or (i.FieldName is not null and d.FieldName is null))

END

How to slve the problem with updated (changed) primary key values?
What is the typical code for audit-trail triggers?

Thanks.